The invention relates to an image display system for transferring image data converted into bit map data from an upper apparatus such as a personal computer or the like to a display apparatus such as a liquid crystal projector or the like and for displaying an image. More particularly, the invention relates to an image display system for displaying an image at a high speed by using an existing bidirectional interface such as a printer interface having a low data transfer speed.
Hitherto, as a display apparatus of a personal computer, a CRT display, a liquid crystal display, or the like is often used. Those display apparatuses are connected to an interface for display of the personal computer and transfer a signal such as an analog RGB signal or the like and display characters or an image. In recent years, many image display systems such that a projector apparatus is externally connected to a personal computer and a presentation is efficiently performed have been put into practical use. In the presentation, a picture plane by bit map data (dot data) such as a document, an image, or the like developed in a frame memory of a personal computer is transferred to a projector, thereby allowing the same picture plane contents to be projected and displayed on a screen. The projector is generally constructed by: a light source such as a halogen lamp or the like; an optical system such as liquid crystal panel, mirror, lens, etc.; and the like. In a display driving, an image to be displayed on the liquid crystal panel is drawn, the image is projected to the screen by the light which is irradiated and transmitted from the back side by a light source lamp, and the projected image is magnified and displayed. As a liquid crystal panel, a TFT (Thin Film Transistor) liquid crystal panel of the active drive type or the like is used.
However, at present, the number of pixels which can be displayed by the TFT liquid crystal panel used in the projector is equal to a value within a range about from (640.times.480) pixels to (1280.times.1024) pixels. Therefore, when a document or a table of the picture plane of the personal computer is magnified and projected by the projector, since the number of pixels is small, it is difficult to read the document, table, or the like. In the presentation using the projector, consequently, the image such as a document or the like of the personal computer cannot be used as it is and there is a complexity such that a document image or the like in which a character size is magnified has to be newly formed so as to enable the image to be clearly seen when it is magnified and displayed by the projector. According to the existing projector, since the same picture plane as the display picture plane of the personal computer is projected and displayed, auxiliary information necessary for retrieval, switching, and the like of the picture plane cannot be obtained. There is, consequently, a problem such that it is difficult to select and explain the picture plane during the presentation or at the time of questions and answers. Further, since only one picture plane can be displayed by the existing projector, even if the operator wants to insert a plurality of picture planes such as documents, graphs and the like into one picture plane and to simultaneously display them onto the screen, there are problems such that since the number of pixels per picture plane is small, the picture quality further deteriorates and, in order to maintain picture quality, the only way is to switch the picture plane and an efficiency of the presentation is bad.
To solve such a problem of the small number of pixels of the liquid crystal panel of the projector, it is considered to use, for example, a phase transition type liquid crystal panel which realizes a number of pixels and a high resolution. As compared with a liquid crystal display panel of the active matrix driving type such as a TFT liquid crystal or the like, since the phase transition type liquid crystal panel is of the simple matrix driving type and has a simple structure, it is suitable for realizing a number of pixels and a high precision. At present, the phase transition type liquid crystal panel of the A4 size having (2500.times.3500) pixels or more has been put into practical use. The phase transition type liquid crystal panel has a structure such that transparent line electrodes and data electrodes are arranged in a matrix form and a liquid crystal material is sandwiched between the electrodes at an intersecting position of both of them. To drive the matrix-form electrodes by applying voltages, two driver circuits of a line electrode driver and a data electrode driver are provided. In a display operation of the phase transition type liquid crystal panel, the line electrode to be displayed is selected and a voltage is applied by the line electrode driver. On the other hand, the data electrode driver applies voltages according to the pixels to be displayed and the pixels not to be displayed in the pixel data to be written. The liquid crystal of each pixel on the selected line electrode becomes a cholesteric phase in which an incident light is scattered when no voltage is applied, so that "black" in which a light transmittance is low is displayed. On the contrary, when a voltage is applied, the liquid crystal becomes a nematic phase in which a light transmittance is high, so that transparent "white" is displayed. As mentioned above, by displaying in accordance with a difference between the light transmittances and sequentially repeating similar processes with respect to all lines, the display of the whole picture plane is accomplished. The phase transition type liquid crystal panel has a memory function. By applying a holding voltage, the display data which has once been written is displayed as it is. The details of the phase transition type liquid crystal panel have been disclosed in, for example,
Phase transition type projection displays (Display and Imaging, Vol. 1, No. 1, pp. 61-69, 1992) PA1 A5-M pixel overhead projection display utilizing a nematic-cholesteric phase-transition liquid crystal (Journal of the Society for Information Display, Vol. 1, No. 1, pp. 43-49, 1993)
According to the phase transition type liquid crystal panel as mentioned above, although the resolution is high and the light transmittance is high and the screen is bright, there are problems such that the display speed is slow and the rewriting time of the picture plane is slower than that of the TFT liquid crystal panel. Namely, it takes tens of milliseconds to rewrite from black (scattering state) to white (transparent state) and, contrarily, a writing time of a few milliseconds is required to rewrite from white (transparent state) to black (scattering state). For example, it takes about 20 seconds to display an image after the phase transition type liquid crystal panel of (2500 lines.times.3500 pixels) was initialized. To solve the above problem such that the rewriting time of the picture plane of the phase transition type liquid crystal panel is slow, in the conventional display control, data of a previous picture plane and data of a new picture plane are always compared and only the different portion is rewritten, thereby reducing the rewriting time (refer to JP-A-61-138991, JP-A-2-217893, JP-A-7-5845, etc.). In the conventional display control in which only the different portion between the new and previous picture planes is rewritten, there is an effect when a picture plane having continuous contents like a motion image is switched. In a liquid crystal projector for mainly displaying a still image for the purpose of presentation or the like, however, there are many cases where the contents of the picture plane are exactly different every time. Even if only the portion different from the previous picture plane is rewritten, a reduction of the rewriting time cannot be expected. It is necessary to improve such a point.
Further, in case of connecting the projector to a personal computer as an external display apparatus and transferring and displaying picture plane data, which interface is used becomes a problem. An interface for display of the personal computer transfers a signal such as an analog RGB signal or the like and displays characters or an image. However, although the interface for display is an analog interface and has a high speed, it can perform only the unidirectional transfer and cannot transfer a signal from the display apparatus to the personal computer. The display interface, therefore, cannot be used for the projector which needs a bidirectional transfer. As another interface for the personal computer, a parallel interface such as IEEE 1284 or the like to connect a printer apparatus is known. The parallel interface is a digital bidirectional interface and transfers bit map data, so that it is suitable to transfer image data to the projector. In case of transmitting data to a printer by, for example, a centronics interface or the like known as a bidirectional interface for printer, a transfer speed is equal to about 100 kbytes/sec. Since A4 monochromatic data having a resolution of 300 dpi corresponding to a display with a number of pixels of the phase transition type liquid crystal panel has a capacity of about 1 Mbytes, it takes a time of about ten seconds to transfer image data. The printing speed of a printer for a personal computer corresponds to a value within a range from 30 seconds to about one minute per sheet of paper of the A4 size in case of, for example, an ink jet printer. No problem occurs about the data transfer speed. In the display apparatus of the projector or the like, even in case of the phase transition type liquid crystal panel of a slow display speed, one picture plane can be displayed in a few seconds. In the bidirectional interface for printer, there is a problem such that the display speed is limited by the transfer speed. Therefore, the projector cannot be connected to the personal computer by using the printer interface equipped as a standard device, an exclusive-use interface of a high transfer speed has to be additionally provided, and the bidirectional interface for the printer lacks generality. Further, even if the display speed of the phase transition type liquid crystal panel is improved, the data transfer speed in the printer interface cannot follow the display speed and a problem such that the display speed is limited by the data transfer speed occurs.